climatological features of strong winds caused by

Climatological Features of Strong Winds Caused by Extratropical Cyclones around Japan

HIDETAKA HIRATAa

a Faculty of Geo-Environmental Sciences, Rissho University, Kumagaya, Japan

(Manuscript received 20 July 2020, in final form 8 February 2021)

ABSTRACT: We examined the climatological features of strong winds associated with extratropical cyclones around Japan

during 40 seasons between November and April from 1979/80 to 2018/19 using reanalysis data. Our assessments revealed that

the extratropical cyclones caused most of the strong winds around Japan (80%–90%). Notably, the contribution of explosively

developing extratropical cyclones is larger (70%–80%). The strongwinds aremainly related to the warm conveyor belt (WCB)

and cold conveyor belt (CCB) inside the explosive cyclones. Moreover, the strong winds tend to be distributed widely over the

southwestern quadrant of the cyclones. This is due to the intensification of the horizontal pressure gradient between themature

cyclones and the Siberian high extending from theEurasian continent to Japan.We investigated the regionality of strongwinds

by highlighting the three areas with high frequencies of strong winds: the area around Hokkaido (i.e., the northernmost island

of Japan; area A), and the areas around the Japan Sea side (area B) and the Pacific Ocean side (area C) of the main island of

Japan. The features of the seasonal change in the frequency of the strong winds differ in each area, which reflects the seasonal

change in the activities of the explosive cyclones. Moreover, the CCB, the head of the CCB andWCB, and the CCB andWCB

bring the strong winds to areas A, B, and C, respectively. The timing of the appearance of these windstorms during the life

cycles of typical cases highlighted in this study is consistent with that observed in Europe.

KEYWORDS: Synoptic climatology; Extratropical cyclones; Wind

1. Introduction

Developing extratropical cyclones frequently pass around

Japan during the period between fall and spring (Yoshida and

Asuma 2004; Adachi andKimura 2007; Hayasaki andKawamura

2012; Iwao et al. 2012; Iizuka et al. 2013; Tsukijihara et al.

2019), bringing strong winds, which directly damage buildings

and infrastructure. Moreover, since the cyclone-induced strong

winds are responsible for high waves (Kita et al. 2018;

Saruwatari et al. 2019) and drifting snow (Kawano and

Kawamura 2018), these are involved in the occurrence of

various natural disasters in Japan. Thus, it is important that

we understand the features of strong winds associated with

extratropical cyclones around Japan.

A number of previous studies have focused on extratropical cy-

clones associated with strong winds around Japan. Hirata et al.

(2016, 2018) demonstrated that the surface latent and sensible heat

fluxes from the Kuroshio and Kuroshio Extension can enhance the

near-surface wind through diabatic processes using numerical sen-

sitivity experiments with respect to these heat fluxes. Kawano and

Kawamura (2018) highlighted an extratropical cyclone causing a

severe snowstorm inHokkaido, Japan, inMarch2013 and examined

the influence of the distribution of sea ice in the Sea of Okhotsk on

the cyclone from numerical simulations. They indicated that the

Okhotsk sea ice distribution affected the strong wind distribution

associated with the cyclone by changing the pressure distribution

near the surface. Tsukijihara et al. (2019) studied the relationship

between the frequency of strong winds in Hokkaido, Japan, and

explosively developing extratropical cyclones (i.e., explosive cy-

clones) in winter from 1979/80 to 2016/17 on the basis of reanalysis

data. Their investigations revealed that the increase in strong wind

events in Hokkaido resulted from an increase in the explosive cy-

clonesmoving northward from theKuroshio region toHokkaido. It

is therefore clear that strong wind events around Japan are closely

related to extratropical cyclones. However, the characteristics of

strong winds associated with extratropical cyclones around Japan

have not been sufficiently studied.

Recently, the characteristics of strong winds of extratropical

cyclones have been examined largely through studies of

European windstorms (e.g., Browning 2004; Baker 2009; Baker

et al. 2013; Schultz and Sienkiewicz 2013; Smart and Browning

2014; Martínez-Alvarado et al. 2014; Slater et al. 2017) and

idealized experiments (Baker et al. 2014, Slater et al. 2015).

These studies indicated that the strong winds of extratropical

cyclones are characterized by three low-level jets: the warm

conveyor belt (WCB), the cold conveyor belt (CCB), and the

sting jet. The structure and time evolution of these low-level

jets are well summarized in Fig. 17 in Clark et al. (2005), Fig. 1

in Hewson and Neu (2015), and Fig. 1 in Hart et al. (2017). The

WCB intensifies along the cold front within the warm sector

during the early life stage of the cyclone. The CCB develops on

the cold side of the warm and bent-back fronts from just before

the time when the cyclone reaches its maximum intensity. The

sting jet appears around the tip of the bent-back front during

the stage of the most rapid development of the cyclone (Clark

Denotes content that is immediately available upon publica-

tion as open access.

Hirata’s current affiliation: Faculty of Data Science, Rissho

University, Kumagaya, Japan.

Corresponding author: Hidetaka Hirata, [email protected]

1 JUNE 2021 H IRATA 4481

DOI: 10.1175/JCLI-D-20-0577.1

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and Gray 2018). The WCB and CCB are sub-synoptic-scale

phenomena, while the sting jet is a mesoscale phenomenon.

Note that not all extratropical cyclones are associated with all

the three jets. For instance, previous studies (e.g., Parton et al.

2010; Schultz and Sienkiewicz 2013; Clark and Gray 2018)

pointed out that sting jets are associated with Shapiro–Keyser-

type cyclones (Shapiro and Keyser 1990). While these strong

wind features (WCB, CCB, and sting jets) have been evaluated

in European cyclones, no such study exists for Japan and this

knowledge gap is addressed here.

Although it is known that strong winds of extratropical cy-

clones cause disasters in Japan, our understanding remains lim-

ited with respect to the features of strong winds of extratropical

cyclones around Japan, as noted above. Motivated by this, we

examined the climatological features of strong winds associated

with extratropical cyclones around Japan. The specific objectives

of this study were 1) to quantitatively assess the relationship

between extratropical cyclones and strong wind events around

Japan, and 2) to clarify the characteristics of the strong winds

associated with extratropical cyclones around Japan.

To approach these issues, we utilized the European Centre

for Medium-Range Weather Forecasts (ECMWF) interim re-

analysis (ERA-Interim) dataset (Dee et al. 2011). As will be

shown in section 2a, these data capture the characteristics of

near-surface winds well, around Japan. On the other hand,

sting jets are not represented in the ERA-Interim data due to

being a mesoscale phenomenon (e.g., Martínez-Alvarado et al.

2012; Hewson and Neu 2015). Thus, this study mainly highlights

the synoptic and sub-synoptic strong winds associated with cy-

clones. Despite this limitation, this study is meaningful as a first

step toward understanding the climatological features of strong

winds associated with extratropical cyclones around Japan.

2. Data and methods

a. Data

To examine the relationship between extratropical cyclones

and strong wind events, we used 6-hourly data from the ERA-

Interim dataset (Dee et al. 2011) with a horizontal resolution of

0.758 longitude 3 0.758 latitude, provided by ECMWF. This

study used 10-m horizontal wind, 2-m temperature, total col-

umn water vapor, and sea level pressure (SLP) data. This study

focused on the period between fall and spring (November–April)

when the extratropical cyclone activity is higher around Japan

(e.g., Yoshida and Asuma 2004; Adachi and Kimura 2007;

Hayasaki and Kawamura 2012). We analyzed the 40 seasons

from 1979/80 to 2018/19.

To confirm the reliability of the ERA-Interim data, we

compared the 10-min-averaged wind speed derived from nine

observation stations of the JMA (shown in Fig. 1) with the

ERA-Interim wind speed at a height of 10m for the grid points

nearest these stations (Fig. 2). Additionally, we calculated the

Spearman’s rank correlation coefficient between these two

variables (Table 1). We selected Spearman’s rank correlation

coefficient because the frequency distribution of the wind

speed was not a normal distribution. The ERA-Interim data

capture the characteristics of the wind speed at each station

(Fig. 2), and significant positive correlations were found at all

stations (Table 1). The correlations differ among the stations:

the strongest correlation was at Aikawa (0.72), while the

weakest correlation was at Shionomisaki (0.42). This differ-

ence may be due to the differences in the surrounding envi-

ronment (e.g., topography, altitude, and land use) among these

stations. Those comparisons indicated that the 10-m winds of

the ERA-Interim data accurately reproduce the features of the

near-surface winds around Japan.

b. Algorithm for tracking cyclones

To identify extratropical cyclones, we utilized the tracking al-

gorithm of Tsukijihara et al. (2019). Following their method, we

first searched SLP fields over the East Asia region (208–658N,

1158E–1808) for a minimum point of SLP within a circle with a

300-km radius using the 6-hourly ERA-Interim data (0.758 30.758). If the minimum value was at least 0.5 hPa lower than the

areal-averaged valuewithin a 300-km radius from theminimum, it

was identified as the candidate of a cyclone center. This searchwas

conducted using an interval of 6h for the 40 seasons from 1979/80

to 2018/19. Using the method of Wernli and Schwierz (2006), the

location of the cyclone center 6h later was estimated as follows:

x(t1 6)5 x(t)1 0:75[x(t)2 x(t2 6)] , (1)

where x is the location of the cyclone center, which is indicated

by degree of latitude and longitude, and t is the time in hours.

The nearest cyclone-center candidate at t1 6 within a radius of

600 km from x(t 1 6) was considered as the cyclone center at

t 1 6. Short-lived cyclones (lifetime , 24 h) were eliminated

from our analyses.

FIG. 1. The eight regions of Japan, shown by different colors. The

dots indicate the locations of nine observation stations of the Japan

Meteorological Agency (JMA). The region enclosed by the green

line is highlighted in this study.

4482 JOURNAL OF CL IMATE VOLUME 34


c. Definition of explosive cyclones

To understand the features of cyclones associated with

strong winds in detail, we classified extratropical cyclones into

explosive cyclones and nonexplosive cyclones. To define ex-

plosive cyclones, we used the cyclone deepening rate «, ex-

pressed as

«5pc(t2 6)2p

c(t1 6)

12

sin608

sinuc

, (2)

where pc anduc are the SLP at the center of the cyclone and the

latitude at the cyclone center, respectively. According to pre-

vious studies (e.g., Yoshida and Asuma 2004; Yoshiike and

Kawamura 2009), if the « of an extratropical cyclone exceeds

FIG. 2. Comparisons between 10-min-averagedwind speed derived from the nine observation stations of the JMA (see Fig. 1) andERA-

Interim’s wind speed at a height of 10m of the grid points nearest these stations. In these comparisons, we used 6-houly data for 10 seasons

between November and April from 2009/10 to 2018/19. See text for details.



1 hPa h21, it is considered as an explosive cyclone. In the

original definition of explosive cyclones (Sanders and Gyakum

1980), the time changes in central pressure of cyclones during a

24-h period are used. On the other hand, the method in this

study used those during a 12-h period. The 12-h method can

also extract cyclones rapidly developing over a short period.

We believed that these cyclones are also dangerous because

they cause rapid changes in weathers over a short period. Thus,

this study used the 12-h method.

d. Definition of strong winds

To define strong wind events around Japan, we used the

6-hourly 10-m wind speed data from the ERA-Interim dataset

within the region enclosed by the green line in Fig. 1. We es-

timated the 99th percentile of 10-m wind speed from all data of

the analyzed region during the 40 seasons. Consequently, the

99th percentile of the wind speed was 15.567m s21. On the

basis of this statistic, strong wind events (probability# 1%) are

defined as those with 10-m wind speed exceeding 15.567m s21.

3. Overview of strong winds associated with extratropicalcyclones

Figure 3 shows the frequency distribution of the strong wind

events during the 40 seasons. Note that eight regional names of

Japan used in this paper are indicated in Fig. 1. This map in-

dicates that there are three regions where the strong wind

events frequently occur around Japan. The first region is

aroundHokkaido, the second region is on the Japan Sea side of

Chubu, Kinki, and Chugoku, and the third region is on the

Pacific Ocean side of Tohoku, Kanto, and Chubu. The fre-

quencies of strong wind events were lower around Shikoku and

Kyushu than around the other areas.

To investigate the degree to which strong wind events

around Japan are related to extratropical cyclones, we esti-

mated the probability that the strong wind events occur in as-

sociation with the cyclones (Fig. 4a). We considered strong

wind events occurring within a 1500-km radius from the centers

of cyclones as the cyclone-related events. If a grid point value

satisfies the strong wind criterion (section 2c) within a 1500-km

radius from two or more cyclone centers, we regarded this

situation as one event. As seen in Fig. 4a, the extratropical

cyclones are related to .80% of the strong wind events over

the whole analytical domain. Around Hokkaido, Tohoku, and

Kanto, where the frequencies of strong winds are higher

(Fig. 3), the probability exceeds 90%. These results indicate

that extratropical cyclones are associated with strong winds

around Japan between fall and spring.

To assess the relative contributions of the explosive and

nonexplosive cyclones to the strong wind events, Figs. 4b and

4c show the probability of strong wind events occurring in as-

sociation with the explosive and nonexplosive cyclones, re-

spectively. The probability that the events occur around Japan

in relation to the explosive cyclones is .70% (Fig. 4b). In

particular, this probability exceeds 80% around Hokkaido and

Tohoku. Nonexplosive cyclones account for approximately

20%–40% of the strong wind events around Japan (Fig. 4c). As

described in Table 2, the number of explosive cyclones passing

around Japan (298–478N, 1278–1478E) is smaller than that of

nonexplosive cyclones. However, the strong wind events are

mainly caused by the explosive cyclones rather than the non-

explosive cyclones. This is one of the important features of

extratropical cyclones causing strong winds around Japan.

To determine where the strong winds occur inside cyclones,

their frequency relative to the center of explosive and nonex-

plosive cyclones is shown in Figs. 5a and 5b, respectively.

Within the explosive cyclone system, the strong winds fre-

quently occur over the northwest and southwest quadrants of

the cyclone (Fig. 5a). Specifically, the strong wind frequencies

were the highest around the south and southwest of the cyclone

center (Fig. 5a). To the east of the cyclone center, the middle

frequencies of the strong winds ($100) were observed (Fig. 5a).

Compared to the other quadrants, the middle frequencies of

FIG. 3. Frequency distribution of the strong wind events around

Japan during the 40 seasons.

TABLE 1. Spearman’s rank correlation coefficient between 10-min-averaged wind speed derived from the nine observation stations of

the Japan Meteorological Agency (JMA) (see Fig. 1) and the ERA-Interim’s wind speed at a height of 10m for the grid points nearest

these stations. To estimate these correlations, we used 6-houly data during 10 seasons between November–April from 2009/10 to 2018/19.

An asterisk (*) indicates that the correlation coefficient satisfies a 1% level of statistical significance.

Nemuro Suttu Enoshima Aikawa Chosi Miyakejima Shionomisaki Sakai Makurazaki

0.68* 0.44* 0.55* 0.72* 0.56* 0.65* 0.42* 0.53* 0.52*



the strong winds ($100) spread widely over the southwest

quadrant of the explosive cyclone (Fig. 5a). As for the non-

explosive cyclone, the strong wind events mainly encircle the

cyclone center (Fig. 5b). The frequencies of the strong winds

are significantly lower in the nonexplosive cyclone category

than in the explosive cyclone category. These results also in-

dicated that the explosive cyclones are themain contributors to

the strong winds around Japan. Based on the results illustrated

in Figs. 4 and 5, we specifically focus on the explosive cyclones

in the following paragraphs.

To see the mean structure of near-surface winds associated

with explosive cyclones, we produced composite maps of 10-m

horizontal winds relative to the center of explosive cyclones

related to the strong winds (Fig. 6). The strong wind frequency

and composited meridional winds at a height of 10m are also

shown in Figs. 6a and 6b, respectively. To the east of the cy-

clone center, where the middle frequencies of the strong winds

were observed (Fig. 6a), southerly winds were strong inside the

cyclone (Fig. 6b). Around the northwest quadrant of the cy-

clone, where the strong wind frequencies were relatively high

(Fig. 6a), easterly or northerly winds prevailed (Fig. 6). To the

south and southwest of the cyclone center, where the strong

wind frequencies were the highest (Fig. 6a), westerly winds

dominated. To the south of the cyclone center, meridional

winds transitioned from northerly to southerly winds (Fig. 6b).

Over the southwest quadrant of the cyclone, where the strong

winds frequencies were widely distributed, northwesterly winds

were evident (Fig. 6a).

Next, to examine the characteristics of the strong winds as-

sociated with explosive cyclones, we produced composite maps

of temperature at 2m in height and total column water vapor

relative to the center of explosive cyclones related to the strong

winds (Fig. 7). The composited horizontal winds at a height of

10m are also shown in Fig. 7. To the east of the cyclone center,

the southerly winds associated with relatively high tempera-

ture (Fig. 7a) and moisture content (Fig. 7b) dominated. These

features of the southerly winds correspond well to those of

the WCB (e.g., Carlson 1980; Browning and Roberts 1994;

Madonna et al. 2014). Around the north and west of the cy-

clone center, the easterly and northerly winds associated with

relatively low temperature (Fig. 7a) and moisture content

(Fig. 7a) were observed. These features of the easterly and

northerly winds are consistent with those of the CCB (e.g.,

Carlson 1980; Schultz 2001; Hirata et al. 2019). To the south-

west of the cyclone center, the moisture content is relatively

low, and the temperature transitioned from low to high values.

Moreover, the northwesterly winds prevailed over the south-

west. These features suggest that the head of the CCB is related

to the strong wind around the southwest of the cyclone center.

To the south of the cyclone center, the composited tempera-

ture was relatively high, and the moisture content increased

fromwest to east. The relatively high temperature suggests that

the WCB is related to the strong winds, while the transition of

the moisture content implies that the head of the CCB is also

related to the strong winds. The transition from the northerly

to southerly winds (shown in Fig. 6b) also suggests that both the

WCBandCCB contribute to the strong winds around the south

of the cyclone center; this is discussed in greater detail in

section 4b.

As seen in Figs. 5a and 6a, the strong winds are distributed

widely over the southwest quadrant of the cyclones, and it is

important to consider the reason why this asymmetry occurs.

Yamashita et al. (2012) reported that when an explosive

FIG. 4. Probability of strong wind events occurring in association with (a) all extratropical cyclones, (b) the explosive cyclones, and (c) the

nonexplosive cyclones. If the frequencies of the strong winds are ,200 (see Fig. 3), probabilities are suppressed.

TABLE 2. Number of explosive cyclones and nonexplosive cy-

clones passing around Japan (298–478N, 1278–1478E) and their

percentage of total extratropical cyclones.

Category Number Percentage (%)

Explosive cyclones 1705 35

Nonexplosive cyclones 3134 65



cyclone grew around Japan, a cold continental high in East

Asia, the Siberian high (e.g., Takaya and Nakamura 2005),

often extends from the Eurasian continent to Japan, and thus

the horizontal pressure gradient increases between these two

systems around the southwest quadrant of the cyclone, which

enhances northwesterly geostrophic winds around Japan (see

Fig. 12 in Yamashita et al. 2012).

To confirm this influence of the Siberian high, we produced

the composite map of SLP and geostrophic component of

horizontal wind estimated from SLP relative to the cyclone

center (Fig. 8). We selected explosive cyclones causing strong

winds in their southwest quadrants as the samples for this

analysis. The high pressure is located to the west of the cyclone,

corresponding to the Siberian high. The high extends to the

southwest of the cyclone center, and thus the horizontal pres-

sure gradient intensifies in situ. The region accompanied by the

relatively strong geostrophic winds ($18m s21) is distributed

widely over the southwest quadrant of the cyclone compared

with the other quadrants, which is consistent with the fre-

quency distribution of the strong winds shown in Fig. 5a. These

results indicate that the combination of the explosive cyclone

development and the Siberian high may cause the higher

FIG. 5. Frequency distribution of the strong wind events relative to the center of (a) explosive and (b) nonexplosive

cyclones.

FIG. 6. Composite map of 10-m horizontal winds (vectors) and sea level pressure (SLP; contours) relative to the

center of explosive cyclones associated with the strong winds. The contour interval is 5 hPa. The reference arrow is

10m s21 (shown between the color bars). Frequency distribution of the strong wind events (shading) and com-

posited 10m meridional wind (shading; m s21) are also shown in (a) and (b), respectively.



frequency of strong wind events over the southwest quadrant

of the cyclones.

4. Regionality of strong winds associated withextratropical cyclones

As shown in Fig. 3, there are the three areas where the strong

winds frequently occur around Japan; the area aroundHokkaido,

the area west of Chubu, Kinki, and Chugoku, and the area east of

Tohoku, Kanto, and Chubu. In this section, to deepen our un-

derstanding of the cyclone-induced strong winds, we conducted a

detailed examination of the three areas described above. On the

basis of the frequencies of the strong winds (Fig. 3), we defined

the three areas as shown in Fig. 9. For convenience, these regions

are referred to as A, B, and C. In this section, we highlight ex-

plosive cyclones, since these are related to many strong wind

FIG. 7. (a) Composite map of temperature at 2m in height (shading), 10-m horizontal winds (vectors), and SLP

(contours) relative to the center of explosive cyclones associated with the strong winds. The shading interval 3K,

and the contour interval is 5 hPa. The reference arrow is 10m s21 (shown between the color bars). (b) As in (a), but

for total column water vapor. The shading interval is 3 kgm22.

FIG. 8. (a) Composite map of sea level pressure (SLP) (contour) relative to the center of explosive cyclones

associated with strong winds in their southwest quadrants. The shading interval 5 hPa. (b) As in (a), but for the

geostrophic component of horizontal wind estimated from SLP. The shading interval is 4m s21.



events around Japan, as described in section 3.Weprovide results

derived from climatological and prototype analyses in sections 4a

and 4b, respectively.

a. Climatological analysis

We first surveyed the seasonality of the frequency of strong

winds in areas A, B, and C. Figures 10a–c show the frequency

of the strong wind events from November to April in areas A,

B, and C, respectively. The frequency corresponds to the

number of grid points satisfying the criterion of the strong wind

divided by the total number of grid points in each area. As can

be seen, the seasonality in the three areas differs. The fre-

quency in November is higher in areaA (Fig. 10a) compared to

the other areas (Figs. 10b,c). The frequency in area A reaches

its peak in December and then subsequently decreases, and is

slightly higher in March than in February, which is also a

unique characteristic of area A (Fig. 10a). In April, the fre-

quency drastically decreases in area A (Fig. 10a). Although the

frequency in area B is low in November, it rapidly increases

and reaches themaximum inDecember (Fig. 10b). Subsequently,

the frequency gradually decreases until April (Fig. 10b). As with

area B, the frequency suddenly increases from November to

December in area C; its peak is observed in January (Fig. 10c).

Although the frequency decreases from January to March, the

values are almost the same (Fig. 10c), whereas from March to

April the frequency rapidly decreases (Fig. 10c).

The seasonal change in the frequency of the strong winds, as

shown in Fig. 10, corresponds well to the seasonal change in the

frequency of the explosive cyclones shown in Fig. 11. In

November, high cyclone densities are observed around the

northernmost part of the Japan Sea and the Okhotsk Sea, or

the western and northern parts of area A (Fig. 11a). This ob-

servation is consistent with the higher frequency of the strong

wind events in November in area A (Fig. 10a). In December,

the cyclone densities around the southern part of the Japan Sea

suddenly increase (Fig. 11b), which corresponds to the rapid

increase in the frequency of the strong wind events in area B

(Fig. 10b). Moreover, the cyclone densities also increase

around Kanto in December (Fig. 11b). This corresponds to the

increase in the frequency of the strong wind events in area C in

December (Fig. 10c). The cyclone densities over the Sea of

Japan gradually decrease from December to April (Figs. 11b–

f), which corresponds well with the change in the strong wind

frequency of area B (Fig. 10b). On the other hand, the higher

densities of the cyclones were maintained from December to

March around the Pacific Ocean side of Japan (Figs. 11b–e).

This is similar to the seasonal transition of the strong wind

frequencies in area C (Fig. 10c). Focusing on the cyclone

densities around the Sea of Okhotsk, we can see that those are

higher in March than in February (Figs. 11d,e). This difference

in the cyclone densities corresponds well to the difference in

the strong wind frequency in area A between February and

FIG. 9. Study areas A, B, and C, shown by orange, blue, and light

green shading, respectively.

FIG. 10. Monthly frequencies of the strong wind events in areas (a) A, (b) B, and (c) C.



March (Fig. 10a). The cyclone densities around Japan drasti-

cally decrease from March to April (Figs. 11e,f), which re-

sembles the seasonal reduction of the strong wind events in all

areas (Fig. 10).

To investigate the features of the strong winds caused by

explosive cyclones in areas A, B, and C, we produced the

frequency map of the strong winds relative to the cyclone

center with respect to each area (Fig. 12). In area A, higher

FIG. 11. Frequency distributions of explosive cyclones in (a) November, (b)December, (c) January, (d) February, (e)March, and (f) April.

FIG. 12. Frequency distributions of the strong wind events relative to the center of the explosive cyclones in areas (a) A, (b) B, and (c) C.



frequencies are found over the northwest and southwest

quadrants of the cyclone (Fig. 12a). This distribution of the

strong winds corresponds well to the feature of the CCB. In

area B, higher frequencies are seen to the south of the cy-

clone center (Fig. 12b), and this distribution of the strong

winds resembles the feature of the WCB. Additionally, part

of the frequencies to the south of the cyclone center may

include the influence of the tip of the CCB, which is further

discussed in section 4b. The relatively low frequencies are

also observed to the northwest of the cyclone center in area B

(Fig. 12b), which may derive from the CCB of cyclones lo-

cated over the Pacific Ocean. In area C, the high frequencies

of the strong winds appear from the southwest of the cyclone

center to the east (Fig. 12c), which may reflect the influence

of both the WCB and CCB. Moreover, relative high fre-

quencies, between 60 and 100, are observed to the north,

northwest, and west of the cyclone center, which is consistent

with the feature of the CCB. Additionally, the middle fre-

quencies, between 40 and 80, extend meridionally over the

southeastern quadrant of the cyclone in area C. This strong

wind zone may also be related to the WCB of the cyclones

located over the Japan Sea, which is discussed in detail in

section 4b. Compared to the other quadrants, the middle

frequencies are distributed widely over the southwest

quadrant of the cyclone in all areas, which is consistent

with Fig. 5a.

b. Prototype analysis

To gain further insights into the features of the strong winds

associated with the cyclones around Japan, we conducted an-

alyses of typical cases causing strong wind in areas A, B, and C

(Figs. 13 and 14). Figure 13 illustrates snapshots of 10-m hor-

izontal winds, their magnitude, and SLP when strong wind

events occurred around Japan in relation to six explosive cy-

clones, identified as cases 1, 2, 3, 4, 5, and 6. Case 1 is relevant to

strong winds in area A; cases 2, 3, and 4 are relevant to area B;

and cases 2, 5, and 6 are relevant to area C. The times in Fig. 13

correspond to the times when strong wind events occurred in

each area. Figure 14 displays the time evolution of the central

pressure of each cyclone, wherein the red circles indicate the

times of Fig. 13.

We first examined case 1, which caused damage in area A.

At 1200 UTC 2 March 2013, the cyclone existed to the east of

Hokkaido (Fig. 13a). The strong surface winds in excess of

18m s21 are observed over the northwestern and southwestern

quadrants of the cyclone. These strong winds are consistent

with the CCB and corresponds well to the cyclone composite

strong winds in area A (Fig. 12a).

FIG. 13. Horizontal winds at a height of 10m (vectors), their magnitude (shading), and SLP (contours) at (a) 1200 UTC 2 Mar 2013,

(b) 1200UTC 3Apr 2012, (c) 1200UTC 30Dec 1985, (d) 1800UTC14 Feb 2007, (e) 1200UTC16 Jan 2005, and (f) 1200UTC 13Mar 2014.

The reference arrow is 40m s21 (shown beside the color bar). Winds , 10m s21 are suppressed. The shading interval is 3 m s21. The

contour interval is 5 hPa. The cyclones seen in (a)–(f) are referred to as cases 1–6, respectively.



At 1200 UTC 2 March 2013, case 1 was at the mature stage

(Fig. 14a). Previous studies showed that the CCB associated

with European cyclones tends to develop during their mature

stage (Clark et al. 2005; Hewson and Neu 2015; Hart et al.

2017). Thus, the time of the development of the CCB seen in

case 1 is consistent with that of European cyclones.

We next focused on the cases bringing strong winds in area

B. At 1200 UTC 3 April 2012 and 1200 UTC 30 December

1985, the centers of both cases 2 and 3 are in almost same

position to the north of area B (Figs. 13b,c). However, the

features of the strong winds of the two cyclones differ. The

northwesterly and westerly winds of case 2 (Fig. 13b) caused

the strong winds in area B at 1200 UTC 3 April 2012. On the

other hand, the southwesterly winds of case 3 (Fig. 13c)

brought strong winds in area B at 1200 UTC 30 December

1985. The features of the strong winds associated with cases 2

and 3 correspond well to the features of the CCB and WCB,

respectively.

The times 1200UTC 3April 2012 and 1200UTC30December

1985 were the times of the late development stage of case 2

(Fig. 14b) and of the middle development stage of case 3

(Fig. 14c), respectively. Studies of European cyclones showed

that the CCB and WCB appear at the late development stage of

the cyclone, and that the WCB intensifies at the middle devel-

opment stage of the cyclone, while the CCBdoes not occur at this

stage (Clark et al. 2005; Hewson and Neu 2015; Hart et al. 2017).

Thus, the timing of the occurrence of the windstorms associated

with cases 2 and 3 corresponds well to that of European wind-

storms (Clark et al. 2005;Hewson andNeu 2015;Hart et al. 2017).

As shown in Fig. 13d, the center of case 4 is located to the

west ofHokkaido at 1800UTC 14 February 2007, at which time

the Siberian high extended from the continent to the western

part of Japan. Consequently, the horizontal pressure gradient

was enhanced between the low and high pressure systems over

the southwestern quadrant of the cyclone, which induced the

strong southwesterly winds over area B.As discussed in the last

paragraph in section 3, this combination of the explosive cy-

clone and the continental high appears to be a cause of the

higher frequencies of the strong winds over the southwestern

quadrant of the cyclones seen in Figs. 5a and 12. The life stage

of case 4 at 1800 UTC 14 February 2007 is the mature stage

(Fig. 14d). This lower pressure associated with the mature cy-

clone is favorable for the intensification of the horizontal

pressure gradient.

Next, we examined the cases causing strong winds in area C.

At 1200 UTC 16 January 2005 and 1200 UTC 13 March 2014,

cases 5 and 6 existed over the ocean to the east of Kanto

(Fig. 13e) and over Kanto (Fig. 13f), respectively. The north-

easterly, northerly, and southwesterly winds of case 5 and the

southwesterly winds of case 6 were responsible for the strong

FIG. 14. (a)–(f) Time evolution of the central pressure of cases 1–6, respectively. Red circles indicate the times of Fig. 13.



wind events in area C. The features of the strong winds of cases

5 and 6 correspond well to those of the CCB and WCB. Thus,

the CCB and WCB appear to influence the climatological

distribution of the strong winds in area C (Fig. 12c). In both

cases 5 and 6, weak wind areas were found to the west of the

cyclone center over the main island of Japan. This may be due

to an increase in the surface friction over land, which is dis-

cussed in section 5. Focusing again on the strong winds asso-

ciated with case 2 (Fig. 13b), the southerly strong winds,

corresponding to the WCB, flow over area C, although its

center is situated over the Japan Sea. The relatively high fre-

quencies of the strong winds over the southeastern quadrant of

the cyclone (seen in Fig. 12c) likely reflect the influences of the

WCB of the cyclones situated over the Sea of Japan.

The times 1200 UTC 16 January 2005 and 1200 UTC

13March 2014 are the mature stage of case 5 (Fig. 14e) and the

middle development stages of case 6 (Fig. 14f), respectively. As

with case 1, the timing of the appearance of the CCB of case 5

corresponds to that of European windstorms. Moreover, as

with case 3, the appearance time of the WCB of case 6 is also

consistent with that observed in European windstorms.

On the basis of the results obtained in section 4, the WCB

and CCB (and their associated features) account for the strong

winds around the cyclone center in areas A, B, and C. The

timing of the occurrence of theWCB and CCB associated with

the Japanese cyclones is very similar to that of European

windstorms. Moreover, the enhancement of the strong winds

over the southwestern quadrant appears to be due to the

combination of a mature cyclone and the Siberian high ex-

tending from the continent to Japan. This is a unique charac-

teristic of the strong winds associated with the cyclones around

Japan, which is related to the geographical feature that Japan is

located to the east of the Eurasian continent.

5. Summary and discussion

In this study, we examined the climatological features of

strong winds caused by extratropical cyclones around Japan

during 40 seasons between November and April from 1979/80

to 2018/19 using the ERA-Interim dataset. First, we quantita-

tively assessed the contribution of extratropical cyclones to

strong wind events, which showed that a substantial portion of

the strong wind events (80%–90%) is related to extratropical

cyclones (Fig. 4a). The contributions of the explosive cyclones

(70%–80%) are larger than that of the nonexplosive cyclones

(20%–40%) (Figs. 4b,c). This study is the first to quantitatively

illustrate the close relationship between the strong winds and

the extratropical cyclones, especially the explosive cyclones,

around Japan.

Investigations of the characteristics of the strong winds as-

sociated with extratropical cyclones around Japan revealed

that theWCB and theCCB associated with the cyclonesmainly

bring the strong winds around Japan (Figs. 5–7). Although

previous studies reported the relationship between the strong

wind events and the WCB and the CCB around Europe (e.g.,

Clark et al. 2005; Hewson and Neu 2015; Hart et al. 2017), this

relationship around Japan was uncertain. To the best of our

knowledge, this study is the first to clearly show that the WCB

and CCB are responsible for the strong wind events around

Japan. Moreover, we found that the frequencies of the strong

winds are distributed widely over the southwest quadrant of

the cyclones, compared to the other quadrants (Figs. 5a and

6a). We pointed out that the higher frequencies over the

southwest quadrant are due to the strong horizontal pressure

gradient between the Siberian high extending from the

Eurasian continent to Japan and the mature cyclones (Figs. 8

and 13d).

We next focused on three areas with the high frequencies of

the strong winds (Figs. 3 and 9), which are the area around

Hokkaido (area A), the area around Japan Sea side of Chubu,

Kinki, and Chugoku (area B), and the area around Pacific

Ocean side of Tohoku, Kanto, and Chubu (area C), and ex-

amined the regionality of strong winds associated with extra-

tropical cyclones. The results showed that the features of the

seasonal change in the strong wind frequencies differ among

each area (Fig. 10). Moreover, the seasonal change in the fre-

quencies of the explosive cyclones explain the seasonal change

in the strong wind frequencies well in each area (Fig. 11). This

again demonstrated the close relationship between the strong

winds and the explosive cyclones around Japan.

The characteristics of the strong winds caused by explosive

cyclones in areas A, B, and C were also examined (Figs. 12 and

13). In area A, the strong winds are associated with the CCB

(Figs. 12a and 13a). In area B, the WCB and the head of the

CCB bring the strong winds (Figs. 12b and 13b,c). In area C,

both the WCB and CCB induce the strong winds around the

cyclone center (Figs. 12c, and 13e,f). Moreover, when cyclones

are situated over the Japan Sea, the associated WCB often

develops over area C, contributing to the occurrence of the

strong winds in area C (Figs. 12c and 13b). In all areas, the

relative high frequencies of strong winds are observed over

the southwest quadrant of the cyclone (Fig. 12).

The results of this study indicated that the strong winds

within cyclones are closely linked to theWCBandCCB around

Japan, similar to those around Europe. Moreover, the hori-

zontal structure and time evolution of the WCB and CCB

around Japan are similar to those around Europe. These sim-

ilarities imply that these features of strong winds associated

with extratropical cyclones are universal. Thus, we presume

that the WCB and CCB contribute to the occurrence of strong

wind events associated extratropical cyclones in other regions.

As the analysis methods used in this study can be applied to

other regions, further studies using our methods can verify this

hypothesis.

The timing of the appearance of the WCB and CCB during

the life cycles of the typical cyclones around Japan (Figs. 13

and 14) also resemble that observed in European cyclones

(Clark et al. 2005; Hewson andNeu 2015; Hart et al. 2017). The

timing of the appearance of the WCB and CCB may reflect

the physical mechanisms of the formation of the windstorms.

The CCB intensifies during themature stage of cyclones. Slater

et al. (2015) showed that the horizontal pressure-gradient force

is the primary cause of the acceleration of near-surface winds

associated with the CCB. During the mature stage of cyclones,

the pressure-gradient force around the cyclone center strengthens

due to the lowest pressure in the cyclone center. Thus, the time



evolution of the pressure-gradient force around the cyclone

center appropriately explains the timing of the development

of the CCB. The WCB develops during the early stage of

cyclones, which suggests that the physical mechanisms of

WCB development differ from that of the CCB. Lackmann

(2002) demonstrated that latent heat release was enhanced

along a cold-frontal precipitation band associated with an

extratropical cyclone, creating maxima of positive potential

vorticity (PV) anomalies along the font in the lower tropo-

sphere. They indicated that the circulation induced by the

cold-frontal PV anomalies strengthened the low-level jet

corresponding to the WCB. The results of Lackmann (2002)

suggest that the evolution of latent heat release along cold fronts

is a key factor determining the evolution of the WCB. Further

studies are required to clarify the effect that latent heat release

along cold fronts has on the evolution of the WCB and why the

WCB develops during the early life stage of cyclones.

Moreover, we found that the higher frequencies of strong

winds were observed over the southwest quadrant of the cy-

clone around Japan (Figs. 8 and 13d). We pointed out that

these higher frequencies are related to the Siberian high. The

Siberian high is an important element of the winter East Asia

monsoon system (e.g., Takaya and Nakamura 2005). Thus, we

speculate that this is a unique feature of the strong winds as-

sociated with extratropical cyclone over the East Asia mon-

soon area. The strong winds over the southwest quadrant of a

cyclone tend to occur during the mature stage of the cyclones

(Figs. 13d and 14d). This is because the lowest pressure in the

mature cyclone is responsible for the strong horizontal pres-

sure gradient between the extending Siberian high and the

cyclone.

This study showed that extratropical cyclones, especially

explosive cyclones, are the key contributors in bringing strong

winds around Japan during the period from fall to spring.

These results suggest that forecasting and monitoring of ex-

plosive cyclones is particularly important for preventing di-

sasters related to the strong winds during the cold season

around Japan. We believe that the distinct characteristics of

the strong winds of the explosive cyclones around areas A, B,

and C, which are revealed in this paper, are useful for regional

disaster prevention in Japan. Moreover, our results suggest

that highlighting long-term variations of explosive cyclones is a

valuable strategy for comprehending long-term variations of

strong wind events around Japan, which is in agreement with

the viewpoint of Tsukijihara et al. (2019).

This study defined strong wind events on the basis of the 99th

percentile of 10-m wind speed from all data within the study

area (see section 2d). Near-surface winds are weaker over land

than over the ocean due to the differences in surface friction

between the land and ocean, as shown in Figs. 13e and 13f.

Consequently, most of the strong wind events were extracted

over the ocean in our analyses (Fig. 3). Thus, the results of this

study mainly showed the features of the strong winds associ-

ated with extratropical cyclones over the coastal areas and

ocean around Japan. Features of strong winds over land are

expected to be more complicated than those over the ocean

because several factors (e.g., topography and land use) over

land may modify the structure of strong winds associated with

extratropical cyclones. This issue will be addressed in detail in

future studies.

As noted in section 1, our analyses were unable to assess the

influences of the mesoscale sting jet. On the other hand,

Shapiro–Keyser-type cyclones, which are associated with

sting jets (e.g., Schultz and Sienkiewicz 2013; Clark and Gray

2018), often appeared around Japan (Takano 2002; Hirata

et al. 2015, 2016), and Hirata et al. (2018) reported that a

strong wind area similar to a sting jet occurred around Japan

(Fig. 4 in Hirata et al. 2018). To further understand the rela-

tionship between strong winds and extratropical cyclones

around Japan, we plan to conduct examinations focusing on

the sting jet using high-resolution cloud-resolving simulations

and a diagnostic method for sting-jet precursor conditions

(e.g., Martínez-Alvarado et al. 2012; Hart et al. 2017).

Acknowledgments. The authors thank the three anonymous

reviewers for their very helpful comments. The author wishes

to thank Eigo Tochimoto and Yousuke Yamashita for offer-

ing helpful suggestions. This work was supported by JSPS

KAKENHI 19K14794.

Data availability statement. The ERA-interim dataset was

provided by ECMWF (https://apps.ecmwf.int/datasets/). JMA

observational data can be downloaded from the JMA website

(http://www.jma.go.jp/jma/index.html).

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